Converting between osgb36 and wgs84

Converting between OSGB36 and WGS84

Introduction

The following pages describe various ways of transforming data from one coordinate system to another, specifically from OSGB 36 to WGS 84.

This is not an exhaustive description; however it covers the main systems used at EDINA (except Cadcorp). There are a lot of very good references available ranging from simple explanations of projections and coordinate systems [1] to quite detailed but readable descriptions [2,3].

Some terminology used in these help pages:

The following are used as interchangeable terms, although in reality they may mean slightly different things:

British National Grid = BNG = OSGB 36 National Grid = EPSG: 27700

WGS 84 = ETRS89 = EPSG: 4326

The most common situation that this applies to is either:

a) Transforming from WGS84 to OSGB36 – for example when collecting data with a GPS (or mobile phone) and you get tracks and points in WGS84 but you want to display them on an Ordnance Survey map in OSGB36.

b) Transforming from OSGB36 to WGS84 – for instance if you want to display OS MasterMap data in Google Earth in KML format you will need to convert to WGS 84 as KML uses WGS 84 by definition.

The Ordnance Survey have provided guidance on how to convert between OSGB 36 and WGS84 contained in the document at [2]. It recommends two approaches.

Either:

1) Where accuracy is not essential, a 7-parameter Helmert transformation can be used that is accurate to +/- 5 metres using the following parameters to go from WGS84 to OSGB36:

- 446.448 + 125.157 - 542.060 + 20.4894 - 0.1502 - 0.2470 - 0.8421

2) Where accuracy is important the National Grid Transformation, OSTN02 should be used. It has an accuracy level of less than a metre (consistent with the accuracy of the original data).

The following table shows a summary of the accuracy statements associated with various transformations available in FME/ArcGIS/PROJ.4

Figure 1: Accuracy comparison

From: http://communityhub.esriuk.com/journal/2012/4/10/understanding-which-transformation-to-choose-in-arcgis.html

Transformations in specific GIS systems

The following sections detail how different systems implement coordinate transformations and suggest how they can be set to use the correct method.

1) Transformations to OSGB36 and WGS84 in FME

2) Transformations to OSGB 36 and WGS 84 in ArcGIS

3) Transformations to OSGB36 and WGS84 in GDAL/OGR

If you need to download the OSTN02 transformation it can be accessed at: OSTN02 data file

Conclusion

From an analysis of different systems it can be seen that the default behaviour in most systems does not provide an acceptable result if the aim is to achieve and maintain the highest level of accuracy. In most systems it is relatively simple to change the default behaviour and this should be done where appropriate.

Unless there are good reasons otherwise, the OSTN02 transformation should be used wherever possible.

Beware, it is very easy to get it wrong! Great care should be taken to check that the transformation is working correctly and that the signs of any parameters is correct. Never accept the default transformation unless you know it is correct for your purpose!

References

[1] eLearning module on Coordinate systems and projections: http://digimap.edina.ac.uk/webhelp/training/training.htm#elearning/eLearning.htm

[2] Ordnance Survey – A Guide to coordinate systems in Great Britain

http://www.ordnancesurvey.co.uk/docs/support/guide-coordinate-systems-great-britain.pdf

[3] Seazone: Introduction to Coordinate Reference Systems

http://www.seazone.com/help-support/SeaZone_Introduction-to-coordinate-reference-systems.pdf

[4] Coordinate Systems in FME: https://knowledge.safe.com/articles/585/using-coordinate-systems-within-fme.html

[5]Geotrans Geographic Translator

http://earth-info.nga.mil/GandG/geotrans/

[6]Defining Coordinate Systems in FME: http://docs.safe.com/fme/html/FME_Desktop_Documentation/FME_Coordinate_Systems/Home_cs.htm

[7]Esri UK – Understanding which Transformation to Choose in ArcGIS: http://communityhub.esriuk.com/journal/2012/3/26/coordinate-systems-and-projections-for-beginners.html

[8] Esri UK – What are my OGC web services displaced by about 120m?: http://communityhub.esriuk.com/journal/2012/10/12/why-are-my-ogc-web-services-displaced-by-about-120-metres.html

[9] Other information about implementing OSTN02 in PostGIS directly is available here:

http://blog.mackerron.com/2012/07/03/ostn02-for-postgis/

[10] Information about transformations in QGIS

http://gis.stackexchange.com/questions/76411/issues-using-geographic-and-projected-coordinate-systems-qgis-2-0

[11] Blog from Frank Warmerdam indicating how transformations are chosen in proj.4

http://fwarmerdam.blogspot.co.uk/2010/03/in-last-few-weeks-i-believe-i-have-made.html

[12] PROJ.4 General Parameters

http://trac.osgeo.org/proj/wiki/GenParms#nadgrids-GridBasedDatumAdjustments